Anyone want to explain lightning to a layman?

[wasteofo2]

I have very little knowledge of physics, I'm 16 and won't be taking physics for another 2 years. But regardless, I'm curious, could somoene explain what exactly causes lightning, what makes it strike etc.? In 7th grade this was actually covered, I think I remember something about differing concentration of electrons between the air/ground and the cloud?

Thanks for any help,
Jacob

 

[Metallicbeing]

Lightning can strike twice...

Lightning fascinates me also. I find myself in awe of its sheer power. Although the concept for why lightning happens is simple, the understanding of real life mechanics behind lightning is getting better. In basic terms, lightning is a discharge between two highly charged areas.
Positive charges are most common in the atmosphere. Sometimes the charges will be reversed. When the difference in potential charge is enough to overcome the “dielectric” barrier between the charged areas, a discharge will occur.

Lightning (electricity) travels from negative to positive charge. Many people don’t realize that lightning will usually originate from the ground and travel to the clouds above. Ground to air strikes can be branch lightning or a simple stroke of lightning (or the more exotic “ribbon” lightning).

Branch lightning originates from many areas of the ground (or high points). A “leader” will reach out into the sky. At the same time, a stroke will attempt to extend towards the ground. As the path of least resistance changes the bolts seem to pause and change course. Finally they meet and the full current is released causing the familiar flash.

Simple strokes of lightning are simple in the fact that they have only a single point of origin. Single strokes of lighting tend to be very powerful. Usually they will strobe. In fact, most of the strikes that we see are actually several strikes.

Air to air strikes can be very dramatic. Most air-to-air strikes are called cloud-to-cloud and “sheet” lightning. This type of lightning is a discharge between two charged areas at different altitudes. Usually it’s two air masses of different temperature moving in two different directions (friction) that give charges to these areas. Sometimes, a charge is created from just differences in humidity (humid being heavier).

Lightning is usually no wider than your thumb, has a potential of a million or more volts, and has thousands of amps running through it. During the main stroke, the plasma fire induced by the extreme surge of electricity through the air causes an explosion. It’s a clap of thunder.

 

[wasteofo2]

Thanks for the explanation, but there are still some things I'd like to know:

- How do differing charges between the ground and upper atmosphere occur?

- Why do differing charges only seem to reach critical mass druing rain storms?

- (general physics question here) When you say that there are charges, do you mean that the molecules of the ground have accumulated a lot of electrons and the atmosphere has lost a lot of electrons? If so, is lightning really a flow of electrons?

 

[Metallicbeing]

Oops, I did it again...

- A charge is built up by friction, that is, any type of activity that involves at least two masses interacting will build up some kind of charge.

- Any change in atmospheric conditions can stir up a good charge. It's just that thunderhead clouds have a better chance at lightning because of their extreme dimensions. The greater the differences (temperature, humidity, pressure), the better the chances for lightning.

- Lightning is indeed the flow of electrons. A charge is where one mass is electron deficient (+ ions), while the other mass has an excess of electrons (- ions). A discharge is where electrons flow to return to their natural equilibrium.

Hope that helps...

 

[Ivan Seeking]

I'm a lightning freak as well. In addition to the great information already given, here are a few more thoughts. I just posted this somewhere else but oh well. There is a constant rain of positive charge that falls to Earth via primarily ice, rain, dust, and other particulates. Globally, this current adds to about a constant 1800 amps and leaves a net positive Earth charge. Lightning acts to restore the balance of charge. Edit: The average worldwide flow of current due to lighting is also 1800 amps.

As highly negatively charged clouds[really just the lower volume of a cloud, the upper part of a cloud is usually positive but this can all be reversed on occasion] pass over land, free charges in the Earth are repelled by the negative cloud above, leaving a net positive region on the earth. Negative charge from the cloud "steps" down towards the positive Earth via the step leader. This happens about 50 meters at a time, with 50 millisecond pauses between each jump. This stepping motion gives lightning its jagged appearance. This is also the intial stroke that establishes and ionized path from the cloud to the earth.

As the step leader reaches earth, the lower part of the now ionized and negatively charged column of air rushes towards the positive Earth in a dramatic, highly energetic flow of charge. As the negative charge leaves, more negative charge is pulled down... an so on. This is known as the return stroke since the bright, active area of the column moves along from the Earth to the cloud. In effect we see the hole moving up as the electrons rush into fill the hole. This is followed by another leader - called the dark leader. This starts the next event in the established trail.

Most lightning strikes consist of four to thriteen individual events. So the notion that lightning never strikes twice is exactly wrong. Lightning almost never strikes once. Up to 42 separate strokes have been detected in a single "bolt". Typically, a dischage reaches a peak current flow of about 10,000 amps, with a total of only 20 coulombs of charge actually delivered due to the short, 3 millisecond or so duration of each return event.

 

[Ivan Seeking]

One more thought. Even though as mentioned, we believe that friction is responsible for the separation of charge within a cloud, the exact mechanism is not completely understood. Other processes may also play a significant role.

 

[wasteofo2]

Here's another lightning question for you holders of all knowledge:

I'm a lifeguard in an outdoor pool, whenever you can hear thunder/see lightning, everyone must get out of the pool for the duration of the thunder/lightning, and for 30 minutes after the last lightning strike is seen/thunder clap is heard. Obviously, if lightning were to strike the pool with people in it, they'd be fried up pretty well. However, if lightning struck a pool with no one in it, what would happen? The specific pool in question has a metal hand rail leading up the stairs and is anchored into the ground. Would electricity travel up through the metal pole, into the ground and cause a huge explosion of concrete?

 

[Ivan Seeking]

No; not even with a direct hit on the handrail I think. At least I have never heard of lightning causing this much damage to concrete. In a direct hit, trees may explode due to the superheating of the moist core. I have also heard of [first hand] nails being pulled out of the floorboards and walls in a house by the magnetic field... In very rare cases lightning has even blown a hole or two in a wall or roof, but I don't think I have ever seen or heard of concrete being blown apart. Many objects set in concrete, like flag poles, are hit frequently.

 

[Ivan Seeking]

A related thought from a high school physics field trip. The Feather River Canyon in northern California has a series of power houses, with each one older than the last as one travels upstream. One of the oldest houses used large copper pipes, I think about a foot in diameter, for the main conductors coming directly out of the generators. I don't remember the exact generator voltage used originally, maybe 112KV or so but at this point I'm only guessing. At one point the conductors lined either side of a narrow corridor that was definitely intimidating, but well shielded and passable. Years ago, before aluminum ladders came along, the copper pipes were unprotected less the handrails that line the corridor. One day in the late 40s or early 50s, I think, a maintenance worker who who had one of the first generation aluminum ladders attempted to traverse the long narrow corridor. He allowed the ladder to turn and make contact with one of the pipes. At the time that I was there in the late 70's, a hole, really more like a 2 ft diameter, shallow crater could still be seen in the concrete where he last stood. We were told that for all practical purposes there were no remains. He was the dead short to ground.